Just annoying, and if you drive a bunch of different vehicles it’s easy to make a mistake on your actual speed, seems like it should be pretty easy for companies to build an accurate speedometer these days.
I'm sure any vehicle company can make an accurate speedometer, but Checkswrecks already provided a pretty logical, coherent argument for why they don't. And more to the point, the facts he presented explain pretty thoroughly why the vehicle makers
shouldn't make one that's accurate from the factory, and should build in a tolerance for both wear on original tires and for different tire sizes that an owner might be using.
As far as having multiple vehicles, I'm not sure of the relevance. Each of those vehicles has a speedometer; each vehicle can tell you if you're adhering to the legal speed limit, whether it's 35, or 55, or 70 or whatever. If one vehicle reads 4 percent over and one reads 6 percent over, so what? If you're driving in, say, a 70 mph zone, you're still safe in both vehicles (legally speaking, that is) if you're doing 70 mph. Is it really that important to be able to be in Car A and say "well in this car i can actually go 73mph and still be safe", and then be in Car B and say "well in this car I can actually go 75 mph and be safe"?. If you just go by the "70 mph" on the speedometer in both vehicles, you're good. And if you're going 80 mph in a 65 zone, you're going to be better off when the Trooper hits you with a radar gun, because your "inaccurate" speedometer means your ticket is going to be for maybe 76 mph and not 80. Not ideal, I know, but it's at least a cost savings, plus maybe the difference between a ticket and a ticket plus suspension.
I honestly can't see the importance in knowing exactly how fast you're going when you're just out driving the family car on a highway, or zipping around on a motorcycle, as long as you know that if you're riding at the posted limit on the speedometer, you're good. If I'm on a nice straight stretch of two lane road through farm country, and my super accurate speedo says I'm going 86.4 mph, how important is it that I know that? I already definitely know I'm going too fast for the conditions (because of things like deer, farm vehicles, hidden driveways) because my speedometer registers way above the posted 55 mph limit. Why is it important to know how much you're exceeding the legal limit, beyond being able to precisely calculate the speeding fine before the ticket is actually issued?